The enzyme phosphoenolpyruvate (PEP) carboxylase orthophosphate:oxaloacetate carboxylase (phosphorylating), EC 4.1.1.31! is best known for its key role in C.sub.4 photosynthesis, which is the fixation of atmospheric carbon dioxide in leaf mesophyll cells. However, PEP carboxylase is expressed in other cell types, such as etiolated leaves and roots, in C.sub.3 was well as C.sub.4 plants, and different isozymic forms have been distinguished. In maize (Zea mays L. subsp. mays; U.S. Public Inbred B73) PEP carboxylase is encoded in the nucleus by a small gene family, and various data indicate the PEP carboxylase isozyme involved in C4 photosynthesis is photosynthesis is encoded by only one member of the family. Furthermore, this gene is substantially divergent in sequence from other family members.
The PEP carboxylase isozyme involved in C.sub.4 photosynthesis is encoded in a gene 5.3 kb (kilobases) long, which has ten exons varying in size from 85 to 999 bp (base pairs). The nine introns vary in length from 97 to 872 bp. The 5' -flanking region includes several short repetitive sequences which have characteristics similar to elements important in light regulated genes from maize and wheat.
Northern blot analysis indicates that the PEP carboxylase isoenzyme involved in C.sub.4 photosynthesis is expressed in a number of tissues including young green leaves, mature green leaves, inner leaf sheaths, tassels and husks. The isozyme is not expressed in seedling stems, roots or seeds. The pattern of expression indicates that the regulatory region and promoter of the gene encoding this isoenzyme can be useful in controlling the expression of other genes in plants. Since the isozyme is not expressed in seeds, but are expressed in green tissue of the plants, it is possible to clone genes, under the control of the promoter, without concern that the product of the cloned gene will end up in food products. Therefore, valuable traits can be transferred to the maize plant without adulterating the food products produced by the plants.